Apparatus is already known for measuring the peak flow rate of exhalation, in which the subject blows into a measurement container to move a piston in the container against the force of a spring, the displacement of the piston progressively uncovering an open slot in the container wall through which the subject's exhalation escapes. The rate of exhalation determines the movement of the piston and the maximum displacement of the piston is thus related to the peak exhalation flow. In British Patent Application No. 2238130 such an apparatus is described in which data processing means are provided for the pressure values measured, and the disclosure of that earlier application is incorporated herein by reference.
Peak flow meters have been used for many years by medical staff to measure lung function. More recently, therapeutic techniques have involved the subjects themselves in measuring their lung function and this has given rise to problems not encountered during clinical use. It is not always easy to ensure that a subject has properly recorded the results he has achieved using the apparatus, and some subjects may use the apparatus in ways that gives false results.